Why is Ukraine so important to Russia?The two neighboring countries have been intertwined for over 1,000 years of tumultuous history. Today, Ukraine is one of Russia's biggest markets for natural gas exports, a crucial transit route to the rest of Europe, and home to an estimated 7.5 million ethnic Russians — who mostly live in eastern Ukraine and the southern region of Crimea. (All told, about 25 percent of Ukraine's 46 million people claim Russian as their mother tongue.) Russia lacks natural borders like rivers and mountains along its western frontier, so "its leaders have traditionally seen the maintenance of a sphere of influence over the countries around it as source of security," said David Clark, chairman of the Russia Foundation, a think tank. That's especially true of Ukraine, which Russia regards as its little brother. "Everybody knows that Ukrainians are Russians," said Kremlin adviser Sergei Markov. "Except for the Galicians" — a reference to the Ukrainian-speaking residents of western Ukraine.

Why do Russians see Ukraine as theirs?It's partly because both nations trace their roots back to the first East Slavic state, Kievan Rus, which stretched from the Baltic to the Black Sea from the 9th century to the mid-13th century. This medieval empire was founded, oddly enough, by Vikings — "Rus" is the Slavic word given to the red-haired Scandinavians — who swept down from the north in the 9th century, conquered the local Slavic tribes, and established their capital at Kiev. The kingdom converted to Eastern Orthodox Christianity in 988, laying the foundation of the modern Russian church. A French bishop sent to Ukraine reported, "This land is more unified, happier, stronger, and more civilized than France herself." But in the 13th century Kiev was devastated by Mongol invaders, and power shifted north to a small Rus trading outpost called Moscow.

What happened to Ukraine after Kievan Rus fell?Its territory was carved up by competing powers, who prized the fertile plains and rich, dark soil that later earned Ukraine the nickname "the breadbasket of Europe." Catholic Poland and Lithuania dominated the country for hundreds of years, but by the end of the 18th century Imperial Russia had grabbed most of Ukraine, except for Galicia, which was controlled by the Austro-Hungarian Empire. The czars referred to their dominion as "little Russia" and tried to crush surging Ukrainian nationalism in the 1840s, banning the use of the Ukrainian language in schools.

How did Ukraine break away?The first independent Ukrainian state was declared in Kiev in 1917, following the collapse of the Russian and Austro-Hungarian empires at the end of World War I. That independence was short-lived. The new country was invaded by Poland, and fought over by forces loyal to the czar and Moscow's new Bolshevik government, which took power in Russia's 1918 revolution. By the time Ukraine was incorporated into the Soviet Union in 1922, its economy was in tatters and its populace starving. Worse was to come. When Ukrainian peasants refused to join collective farms in the 1930s, Soviet leader Joseph Stalin orchestrated mass executions and a famine that killed up to 10 million people. Afterward, Stalin imported millions of Russians and other Soviet citizens to help repopulate the coal- and iron-ore-rich east. This mass migration, said former U.S. Ambassador to Ukraine Steven Pifer, helps explain why "the sense of Ukrainian nationalism is not as deep in the east as it is in the west." World War II exacerbated this divide.

What happened during the war?When the Nazis invaded Ukraine in 1941, many locals welcomed the Germans as liberators from the Soviets, and tens of thousands even fought alongside them, hoping Adolf Hitler would reward them with an independent state. Later, when the Nazis began using Ukrainians as slave labor, about 2.5 million fought for Stalin's Red Army. The country became one of World War II's bloodiest battlefields. At least 5.3 million Ukrainians died during the war — about one sixth of the population. About 2.25 million of those killed were Jews, targeted by both the Nazis and some Ukrainian collaborators. At the end of the war, Stalin deported tens of thousands of Ukrainians accused of cooperating with the Nazis to Siberian prison camps, and executed thousands more.

When did Ukraine become truly independent?In 1991, more than 90 percent of Ukrainians voted to declare independence from the crumbling Soviet Union. But Russia continued to meddle in the country's affairs. In Ukraine's 2004 presidential election, the Kremlin backed pro-Russian candidate Viktor Yanukovych. Massive fraud in that election sparked the Orange Revolution, which kept Yanukovych from power. The failure of subsequent leaders led to Yanukovych's making a comeback in 2010. But after he canceled a trade deal with the European Union, he was driven from office again last month by pro-Western demonstrators. Despite the world's outrage, Russian President Vladimir Putin is unlikely to let Ukraine leave his country's orbit. "Russia without Ukraine is a country," explains Daniel Drezner, an international politics professor at Tufts University. "Russia with Ukraine is an empire."

Crimea: Khrushchev's mysterious giftCrimea has become a flash point in the struggle between Kiev and Moscow, with Russian troops seizing control of the southern peninsula bordering on the Black Sea. But exactly why this region — which has a majority ethnic Russian population and is home to Russia's Black Sea fleet — ended up as part of Ukraine is something of a mystery. The peninsula had been ruled by Russia for centuries when Soviet leader Nikita Khrushchev suddenly gifted it to Kiev in 1954. Many Russians think Khrushchev was drunk when he signed the Crimea away, while others believe he was trying make amends for the Ukrainian famine. The handover remains deeply unpopular with ordinary Russians, 56 percent of whom view Crimea as Russian territory, far more than feel a claim on Chechnya. "Many see Putin as the one who returned some of Russia's strengths,'' said Denis Volkov, an independent Russian pollster. "I think he will use this idea of the loss of the Soviet Union to drum up support with Crimea."